This application claims the priority of German Patent Application No. 100 30 383.8, filed Jun. 21, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention proceeds from a method and an appliance for the non-thermal drying of articles, in particular motor vehicle bodies, freshly painted with a water-based paint, in which the articles to be dried are introduced into a drier housing, the surfaces of the articles are subjected to dry air in the drier housing by forced convection and, after the paintwork has dried, the articles are discharged from the drier housing, after the air has passed through the drier housing, after the air has passed through the drier housing the moisture-laden air being sucked away from the drier housing, the moisture contained being extracted from said air in an air drying device by condensation and the air thus being dried to a predetermined circulating-air target moisture and being blown into the drier housing again. The method also relates to an appliance for carrying and the method. German Patent Document DE 196 44 717 A1 describes a method and appliance of this general type
Articles, in particular vehicle bodies, painted as standard are painted in a plurality of coats, usually water-based paints being employed at least for the priming and the top-up painting for reasons of environmental protection. However, these paints cannot be applied wet-on-wet, but, instead, before each subsequent application of a further coat of paint, the coat of water-based paint previously applied has to be dried to specific maximum permissible residual moisture of about 5 to 10% by weight. During the subsequent thermal stove-enamelling of the entire composite structure of the paints, higher residual moisture would result in the paint bursting open, because the quantity of water vapor released from the residual moisture cannot defuse through the composite structure quickly enough.
The intermediate drying of the individual coats, particularly of water-based paints, has hitherto been carried out thermally in a drying tunnel, in which the paint on the vehicle bodies is heated by hot circulating air and/or by radiant-heat emitters and the water contained in it is thus evaporated carefully. The moisture-laden spent air is extracted as a whole from the drying process, if appropriate after a recovery of heat. This is a highly energy-intensive drying operation, especially since not only the drying air, but also the vehicle bodies, have to be continuously first heated to high temperature values and then cooled again.
A less energy-intensive drying method, at least for water-based paints, has therefore been considered, this being described in principle in the publication mentioned in the introduction. This proposal is correct in its approach. A problem which it does not solve arises from a different incidence of moisture on the freshly painted articles. In everyday production, the freshly painted articles are not introduced into the paint drying plant in a time sequence which is always constant, but, instead, it is necessary, in this respect, to allow for a change, for example with interruptions of greater or lesser length or with a recommencement of production.
An object of the invention is to improve the generic method and the corresponding generic appliance to the effect that, even in the event of a different and, above all, rapidly changing incidence of moisture on the painted articles, these can always be dried to a specific target residual moisture and a constant dwell time of the painted articles in the paint drying plant can be nevertheless maintained. Exceeding the target residual moisture in the dried paint, that is to say insufficient drying of the paint, causes the paint to burst open during the subsequent stove-enamelling, thus resulting in time-intensive and cost-intensive additional work and serious logistical problems. On the other hand, the paintwork should not be overdried, because this is unfavorable for a good binding of the coat of paint subsequently applies. Apart from this, overdrying would also be uneconomic in terms of energy and cost. Moreover, a varying dwell time of the painted articles in the paint drying plant cannot be allowed for logistical reasons.
Taking the generic method and the generic appliance as a basis, the object mentioned is achieved according to the invention by a method, wherein the drying performance of the air drying device is adapted as required with a low degree of inertia to a changing infeed of moisture into the drier housing, and wherein with the high condensation performance remaining unchanged, only a fraction of the circulated air which is adapted in size as required, is dried in the air drying device to a residual moisture below the circulating-air target moisture, and the remaining untreated part of the circulated air is led back into the drier housing after being intermixed with the dried part-air stream.
This object is further achieved by providing an appliance wherein a bypass line is provided which branches off from the spent-air stream upstream of the suction blower and bypasses the air drying device and conducts untreated air to the supply-air blower, wherein the suction blower is provided with a variable-speed drive and conveys only a variable fraction of circulated air through the air drying device which amounts to a maximum of about 60% by volume of the total circulated air, wherein the air drying device is dimensioned and designed in such a way that in the case of a maximum loading of the spent air with moisture and with maximum load being applied to the air drying device, the air treated in the latter can be dried to a markedly lower value, preferably half the value, of the circulating-air target moisture, wherein the bypass line and the line emerging at the air drying device and carrying the dried air are combined upstream of the supply-air blower, wherein a controller is provided which processes measurement data from a moisture sensor located on the supply-air side and/or those from a moisture sensor located on the spent-air side and by means of which the conveying capacity of the suction blower can be varied automatically via the rotational drive speed of the latter, in such a way that the desired circulating-air target moisture can be maintained on the supply-air side.
A rapid adaptation of the drying performance of the plant to the changing incidence of moisture is important for reliably maintaining the target residual moisture at least within a permissible tolerance range. According to the invention, rapid adaptation is achieved on the air side by a variation in a dried part-air stream and an untreated part-air stream. In the case of a high incidence of moisture on the painted articles, the operation is carried out with a high dried air fraction; the untreated circulating-air fraction is correspondingly lower. In the case of a low incidence of moisture, the situation is reversed; then, the dried air fraction is reduced in favor of the untreated circulating-air fraction. The air drying device is designed, in terms of its drying performance, in such a way that, even in the case of the maximum dried part-air stream required, the circulating-air target moisture can still be maintained reliably. With the reduced fraction of air to be dried, although the air is overdried, as compared with the abovementioned state, due to the installed drier capacity, this is not harmful because of the intermixing with the untreated fraction; in any event, the desired circulating-air target moisture can be maintained for a short time. Overdrying of the dried air can readily be justified on energy grounds, because the energy balance, even in the case of an overdried fraction, is not appreciably different from that occurring in the design situation. It is merely necessary for a corresponding capacity of a drying performance to be installed once.
Expedient refinements of the invention may be gathered from this description and the claims. Morever, the invention is also explained below with reference to an exemplary embodiment illustrated in the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.